1. Field of the Invention
This invention relates to the field of air compression for a steady flow operating requirement. It also relates to the compression of other gases and will even apply to the pumping of liquids. More particularly, this invention relates to the compressors for superchargers and gas turbine type engines. This disclosure cites the application in a radial flow type compressor but it may be applied, also, to an axial flow compressor.
2. Description of the Prior Art
So far as is known the simple and improved diffuser described and claimed herein has not been known heretofore. To those skilled in the art, the importance of obtaining high compression efficiency without a significant surge problem are generally known. Other inventions cited herein illustrate previous efforts to advance this art. In most compressor applications, the flow capacity of the compressor must be matched closely with the flow capacity of the machinery which uses the fluid being compressed to avoid the surge problem. Some of the most common applications are in superchargers for piston engines and air compressors for gas turbine engines.
The problem of compressor surge resulting from unstable flow in a specific operating regime is well known. One example of a method of overcoming the surge problem is disclosed in C. A. Macaluso Et Al U.S. Pat. No. 3,069,070, Dec. 18, 1962. This method requires a variable geometry diffuser which consists of many moving parts and also requires some compromise from an optimum passage shape, which therefore does not gain as much compressor efficiency and is more expensive. Another example is disclosed in Thompson U.S. Pat. No. 2,399,072, Apr. 23, 1946. This method also requires considerably more complication and expense.
Other patents which describe related devices are as follows:
______________________________________ United States Patents Cited ______________________________________ 3,778,186 12/1973 Bandukwalla 3,333,762 8/1967 Vrana 2,967,013 1/1961 Dallenbach Et Al 2,708,883 5/1955 Keller Et Al 2,596,646 5/1952 Buchi ______________________________________
The above do not include the important improvements which this invention discloses, and which past experience has shown will give significant improvements. This invention discloses a method of calculating and designing diffuser vanes and passages that constitutes an advancement to the centrifugal compressor art.
Some examples of research reports which are associated with the problems described in this disclosure and which lend support to the logic of this invention, are as follows:
______________________________________ Literature References ______________________________________ REF. 1) Abott, I. H., Von Doenhoff, A. E. Theory of Wing Sections, Dover Pub. Inc. REF. 2) Fox, R. W., Flow Regime Data and Design Methods for Curved Subsonic Diffusers, Oct. 1960. Stanford University REF. 3) National Advisory Committee for Aeronautics. Report 1135. Equations, Tables and Charts for Compressible Flow. ______________________________________
REF. 1 presents the performance test results of a large number of airfoil shapes for aircraft wings, in a high velocity air stream at various inflow angles. My invention utilizes this technology, since diffuser vane leading edges are similarly exposed to a wide range of air inflow angles, which is a major cause of performance loss and compressor instability, commonly referred to as surge.
An example of a method of reducing this loss in centrifugal compressor is disclosed in Atkinson U.S. Pat. No. 2,819,012, Jan. 1958. While that invention is intended to improve the impeller performance, the art described in that patent also improves the radial flow velocity distribution leaving the impeller outer diameter. This reduces the variation in inflow angle into the diffuser vanes along the length of the leading edge, thereby improving the diffuser efficiency and reducing the tendency to surge.
REF. 2 is an example of research which presents the results of flow bench laboratory testing on curved diffuser passage shapes, in which is cited the importance of avoiding a high pressure gradient in a diffuser. In nearly all current diffuser designs made for the type compressor to which this invention relates, there is a severe pressure gradient following the entrance to the diffuser passage, which is contrary to these findings. The reason that this problem can readily exist is explained by a study of Table I in REF. 3 which applies to the flow of a compressible fluid such as air. This shows that at Mach 1.0, (100 percent of the velocity of sound) a 1 percent increase in channel area results in a 9 percent decrease in velocity and a 12 percent increase in static pressure (neglecting friction losses). Nearly all current diffuser designs embody an excessively rapid increase in passage area, which is thus undesirable.
This invention takes this phenomenon into consideration and discloses a method of calculating this pressure gradient and gives limiting values for this parameter to prevent early flow separation, thereby permitting operation over a broad range that would otherwise cause a performance loss and surge.